The invention relates to a fluid friction coupling having a housing as a first coupling part and at least one hub rotatably supported therein as a further coupling part, outer plates non-rotatingly associated with the housing, as well as inner plates non-rotatingly associated with the hub(s). The outer and inner plates are alternately arranged in a certain sequence and at least the plates of one of the plate sets are arranged so as to be axially movable. The part of the housing interior not occupied by plates is at least partially filled with a viscous medium, especially silicone oil, and different torques are to be transmitted in the two directions of rotation.
When slip occurs at the directly driven motor vehicle wheels on the road surface with a lower adhesion coefficient, it is desirable to generate a traction force for the further wheels. For generating the necessary torque for passing the traction force onto the non-driven wheels, such drives are frequently combined with fluid friction couplings which, for example, eliminate the differential effect of a so-called open differential at least partially or supply torque directly to the wheels not driven directly.
From DE-OS 33 17 247, for example, it is known to derive a rotary movement for the front wheel drive vehicle from the differential of the front axle and pass it on via the driveline to the rear wheels, with a fluid friction coupling being incorporated into the driveline. If both the rear and front wheels rotate at the same speed or if the speeds of the two axles do not deviate greatly, there is no relative movement in the fluid friction coupling generating a substantial torque. However, if there does occur such a speed differential, the plate sets, which are part of the coupling, move relative to each other. In the process, the fluid contained in the remaining space in the housing is sheared between the plates, with a torque building up as a result. If, for example, the front wheels of the front wheel drive vehicle are subject to slip, a torque is built up and passed to the rear wheels. If the vehicle is braked, with the front wheels possibly being blocked, there also occurs a speed differential and thus a relative movement between the plates. In this condition, the rear wheels rotate faster than the front wheels and a torque trying to block the rear wheels as well is built up. The object is to prevent such a condition because it causes the vehicle to lose its lateral guidance.